1. Field of the Invention
The present invention relates to a recording sheet for pressure measurement, more particularly, to a recording sheet for pressure measurement comprising a regularly embossed support having coated thereon a color former or color formers and a developer, the recording sheet being capable of to measuring simply pressures such as surface pressure, line pressure, etc., with high accuracy.
2. Description of the Prior Art
There has hitherto been an urgent need for mechanically measuring various kinds of surface pressures, line pressures, etc., such as the clamping pressure of bolts in general machinery and tools, the pressed state of brake gears for automoblies, mold pressure at thermoplastic molding and thermosetting molding, clamping pressure at such moldings, adhesive pressure between a resin in a mold and a mold wall at such moldings, molding pressure in molds at such moldings, spring-contact surface pressure, clamping surface pressure in the connections of carburetors, fuel pipes, fuel tanks, etc., in fuel lines, the collision pressure exected on a human body in an automobile collision, line pressure and surface pressure of rubber or plastic rolls, the effective limits of gasket seal, to check the effective state of a gasket seal, the oil pressure in brakes, the contact pressure of gears, the contact surface pressure between solid parts, bolt clamping pressure, checking the contact state of the cock of a cock valve, the contact state of curved surfaces, the contact state of rolls for steel during working, the pressure distribution state of soles of men on a floor, the pressure distribution state between a human body and chair, and others. However, there is no method which can easily measure such pressures or methods, if any, which are useful are very complicated at use.
For example, as a method of measuring pressure such as surface pressure, line pressure, etc., there are known a strain gauge method in which the relation between stress and strain is utilized, a method in which a load meter is used, and a method in which a pressure-sensitive paint (a strain-sensitive lacquer) is used.
However, the use of the strain gauge method for measuring surface pressure, line pressure, etc., requires not only large-scale equipment such as an amplifier, a detector, a recorder, etc., but also a specific technique for operation and complicated calculations to determine the strength of the materials. Furthermore, in this case, the materials measured are required to have smooth surfaces. These matters are described in detail in J. Yarnell; "Strain Gauge", published by Corona Sha.
The method of using a pressure-sensitive paint or strain-sensitive lacquer is liable to be accompanied by uneven coating of the paint or lacquer, and has problems with respect to adhesive properties and operation.
In the case of using a load meter, it is difficult to reduce the size of the measurement system, and, thus, such a system is improper for measuring surface pressure, line pressure, etc.
One method of using a pressure-sensitive laminate for measuring pressure is described in U.S. Pat. No. 3,647,504, but the pressure-sensitive laminate disclosed in this patent is a mono-leaf recording sheet containing different kinds of microcapsules having different wall thicknesses, each containing a marking liquid of a different color. Thus, the kind of microcapsules ruptured differ according to the force applied, which results in releasing marking liquid of a specific color for each applied force or pressure. Hence, the extent of the applied force of pressure can be determined by measuring the color tone formed.
In this method, while the pressure may be measured qualitatively, it is difficult to accuratly measure the pressure.
As one improvement of these faults in the afore-mentioned conventional methods or products, one method has been provided in which a pressure measuring recording sheet is brought into contact with a part to be measured and the sheet is colored by applying pressure thereto, where the pressure applied is determined from the optical density change of the colored and recorded image. In this method, the coloring of the pressure measuring recording sheet by pressure can be obtained by contacting a color former and a developer under pressure at the part to be measured, and the improved point of the method is in that surface pressure, line pressure, etc., at the part to be measured can be measured without the need of large scale equipment, highly specialized techniques or complicated calculations.
However, since the support of the recording sheet used in this improved method usually has a smooth or irregularly roughened surface, the recorded image thus colored by the application of pressure is accompanied by the formation of uneven color density, which makes it difficult to accurately measure the color of the sheet to determine the value of the applied pressure.
That is, when a support having an irregularly roughened surface is used as the support for the recording sheet, the pattern of the irregular roughness of the surface is transferred in the recorded color image to result in uneven color density, but the reason why such an uneven color density is formed even in the case of using a support having a smooth surface as the support for the recording sheet has not yet been clarified. The inventors, however, theorize the reasons to be as follows:
One of reason is considered to be based on the characteristics of the microcapsules which contribute to the coloring of the recording sheet. Namely, it is believed that pressure is directly applied to all microcapsules containing a color former, and rupture of the microcapsules is greatly influenced by the unevenness in the particle size of the microcapsules or unevenness in the wall thickness of the microcapsules. Furthermore, another reason is considered to be the influence of gases such as air, etc., existing at the portion to be measured i.e., when pressure is applied to the smooth surface of the recording sheet at the portion to be measured, the gas existing at the interface between the surface of the recording sheet and the surface of the material to be measured (where the recording sheet is set) locally collects at the interface therebetween, which results in uneven pressure being transmitted to the recording sheet for pressure measurement to form an irregular local unevenness in the optical density of the recorded color image.